The surgical use of high-frequency current dates back to the early 1900's. Tesla coil resonators, in conjunction with spark gaps, produce high voltages at very low currents that can be used to destroy superficial tissue. In spark gap oscillators, the periodic breakdown of the spark gap excites resonant circuits which then generate damped, high-frequency electrical wave form. In electrosurgery, the heat that destroys tissue is not produced by a heated wire, as in electrocautery, but by conversion of high-frequency, electrical energy in the tissue. Current density and duration determine the amount of heat generated and tissue destroyed at and near the electrical arc. Active electrodes have small tips to increase the current density at the surgical site. Electrodes used specifically for cutting have small points or edges to concentrate the electrosurgical current. Coagulation electrodes have larger surface areas. Electrosurgery is a very useful tool and provides very good surgical results, particularly in areas involving capillary beds such as the brain, tissue around the spine, liver, spleen, thyroid and lung tissue. In such organs, electrosurgery is used for simultaneous cutting and coagulation (hemostasis).
High frequencies are used in electrosurgery because they tend not to stimulate the patient's muscles. The ability of electrosurgical current to effect tissue depends on the . duration and density of the current. The greater the current density, the more pronounced will be its heating effect.
Electrosurgery, like many other applications of electricity, require a complete circuit for current flow. The circuit begins at the high-frequency generator within the electrosurgical unit, goes through the active cable and active electrodes to the patient and returns to the generator by way of a return electrode or cable.
The active electrode is small, and concentrated heating near its point of contact with the patient causes cutting or coagulation of tissue. Since tissue heating is not desired where the current leaves the patient to return to the electrosurgical unit, the return electrode has a large area of contact with the patient to provide low current density. If the return electrode does not provide low density, low resistance paths for the current, the current will seek alternative means to return to the electrosurgical unit and complete the circuit. Unless these alternative paths provide low current density, tissue heating and burns can result.
In one kind of electrical surgical unit, the return electrode is a large, electrically-conductive plate placed under the patient's body and in good contact with the body. Thus, the current enters the patient's body through the active electrode and passes through the body to the return electrode to complete the circuit. This is called a mono-polar system. There are alternatives to this kind of design.
A bipolar forcep contains two electrodes and contacts the tissue at two points. Current flows into the tissue at one electrode and back out at the other. The entire circuit pathway within the patient is confined to the small area around the two halves of the forceps, and no large return electrode plate is needed. This is the kind of electrosurgical electrode which is preferably used with the present invention.
In bipolar units, the output is typically not connected to ground. If the isolation is effective, current cannot find its way back to these units through the alternative path to ground. Current must leave the patient through a return electrode or it cannot flow at all. A bipolar unit with good output isolation reduces the hazard of patient burns and alternate grounding points.
Although electrosurgical devices of this kind are very useful, there is always a concern when using such devices to avoid unwanted electrical shock to the patient. Many present electrosurgical device designs do not meet the specifications of standard testing in laboratories. These specifications require in part that when a prescribed voltage is applied between the ground on the chassis of the electrosurgical apparatus and the output connector for the patient electrode, no current will flow for a prescribed period of time. One standard requires that with this kind of electrosurgical coagulator and cutter the voltage that must be applied is approximately 8,000 volts of alternating current. An electrosurgical apparatus of the prior art is shown in FIG. 1.
Referring now to FIG. 1, there is shown an output circuit 10 for an electrosurgical power source of the prior art including a power-drive transformer 12 for introducing a relatively high-voltage, sinusoidal alternating current signal into the output circuit 10 through contacts 11 and 13. Transformer 12 is an iron core, grounded, step-up transformer for significantly increasing the voltage supplied to the secondary. The iron core is grounded through permanent attachment to the chassis of the device which houses the circuitry. Additional circuitry like switches, filters and fuses may be incorporated into the input circuit of the prior art, but they have been omitted from FIG. 1 and this description. A spark gap 14 is connected across the secondary of power-drive transformer 12. Spark gap 14 is chosen so that the spark will break down and become conductive at a voltage needed to achieve the maximum output level. Connected in series with spark gap 14 is a tank circuit including a capacitor 16 and an inductance coil 18 which together provide a resonant circuit which is tuned for a desired frequency. For electrosurgical coagulation, a frequency of 2 mhz has been found to be appropriate. Induction coil 18 of the output circuit actually forms the primary coil of a transformer 20 which is coupled to a secondary coil 22. Transformer 20 is a high-frequency, air-gap transformer. In prior art devices, the secondary coil 22 has provided a variety of taps 24 which may be selectively connected to output terminals 26 and 28 through multi-position switch 30. Capacitor 32 provides a tuned resonant circuit in conjunction with secondary coil 22 which is matched in frequency to that of the resonant circuit formed by capacitor 16 and induction coil 18. This kind of circuit is commonly identified as a Tesla circuit referred to above. It can be seen that if a high test voltage on the order of 8,000 volts is applied from the output terminal 26 to ground 31, switch 30 will have to withstand that full voltage. A switch which is capable of withstanding this kind of high voltage would be extremely expensive and probably also very large in dimension. It is, therefore, useful to design an output circuit which removes the switch from the secondary circuit.
In certain prior art devices the hardware for switch 30 has been grounded to the chassis of the power supply so that the grounding path goes directly through this switch housing to the chassis. In this kind of design the patient can be grounded and subjected to undesired electric shock. It is, therefore, doubly desirable to remove the switch from the secondary circuit so that the secondary circuit can be completely isolated from ground and so that the secondary can be heavily insulated.